The motivation to develop terahertz technologies is, in large part, to enable the exploitation of the unique properties of terahertz radiation [1]}, [2]}. Terahertz radiation combines the ability to deeply penetrate many common materials such as plastics, cloth and paper whilst also being non-ionising, thereby providing a safe alternative to x-rays for some applications. Furthermore, terahertz radiation provides a useful tool for materials identification, as the absorption bands of many technologically-useful materials and molecules fall within the terahertz range. Terahertz radiation is strongly absorbed by polar liquids such as water, making it useful for passive hydration monitoring [3]}. However, the same property means that terahertz radiation is strongly attenuated in the Earth's atmosphere because of the water vapour content [4]} and is therefore unsuitable for long-distance free-space communication technology. Nevertheless terahertz-frequency fields are proposed for local data transmission within 10 metres [5]} and for wireless communications at high altitude and in space (aircraft to satellite, or satellite to satellite) [6]}. The properties of terahertz radiation enable a multitude of other applications ranging from security screening [7]}, and non-destructive evaluation for quality control in industry [8]}, [9]}, [10]}, drugs and explosives detection [11]}, [12]}, [13]}, biology and medicine [14]}, [15]}, [16]} to ultrafast spectroscopy [17]}, astrophysics and atmospheric science [18]}, [19]}. Many of these applications involve terahertz imaging as they require the spatial structure of the target material to be recorded [20]}, [21]}.
